Tin-silver coatings are now employed in the electronics and connector industries, replacing the use of pure tin or more expensive pure silver, or gold coatings. Moreover, tin-silver alloys are superior to pure tin (as a replacement for gold) due to the alloy's lower coefficient of friction, which reduces connector insertion forces and prevents connectors from “freezing” together, facilitating multiple insertion cycles. The substrate metal onto which the coating is deposited is typically copper or copper alloy, although other metals may also be used. A tin-silver alloy near the eutectic composition of 96% to 97% tin is usually desired in these applications. In the connector industry, reel to reel plating machines having current densities between 75 and 400 ASF are employed in high speed electroplating processes. The electrolyte must deposit an alloy that does not vary greatly in composition with changes in current density and in bath operation parameters, such as temperature and metal concentrations.
Electro-deposition of alloys is generally more difficult than electro-deposition of pure metals. Differences in the standard reduction potentials of the metals to be deposited cause the difficulties. If the standard reduction potentials are similar, for example tin and lead, electro-deposition presents no problem; the alloys are deposited from simple acid electrolytes and the composition of the deposited alloy is controlled by the relative concentration of the metals in the electroplating solution. If the standard reduction potentials of the metals to be deposited are significantly different, simple acid electrolytes will fail to achieve deposition regardless of the relative concentrations of the metals in the electrolyte. Tin-silver alloys fall into this latter class. Silver has a standard reduction potential of 0.8 V and tin has a standard reduction potential of −0.12 V, indicating that it is much easier to reduce silver to its metallic state than it is to reduce tin to its metallic state. To electro-plate such alloys, it is necessary to complex one or both metals with a complexing agent that brings the standard reduction potentials of the two metals closer to each other.
Another problem arises in tin-silver electroplating: silver immersion plates onto metallic tin surfaces through a displacement reaction between the dissolved silver ions and metallic tin. As a result, silver is reduced at the surface of tin anodes immersed in the electrolyte. Reduction is independent of current flow and results in the continual loss of silver ions from the bath. Silver also immersion plates onto tin-silver cathodes in the absence of current. Complexing silver ions reduces the rate of the displacement reaction but does not eliminate it completely. Silver concentration, bath temperature, and liquid shear rate at the tin surface all influence the rate of the silver displacement reaction. To minimize the displacement reaction, the bath must operate at low temperatures, low silver concentrations, and low liquid agitation around the anodes. These requirements have heretofore prevented high speed electroplating of tin-silver alloys, because high speed electroplating, as used in the electronics and connector industries, is conducted at high metal concentrations, elevated temperatures, and high liquid agitation rates.
Cyanide has been employed as a complexing agent for alloy plating processes; however, the toxicity of cyanide makes it undesirable due to worker safety and waste treatment considerations. Other complexing agents that have been employed with varying low to medium degrees of success include hydantoin (for silver) and gluconate (for tin), see WIPO Patent Publication No. 99/41433 (Toben et al.), alkylsulfonic acids, see U.S. Pat. No. 6,998,036 (Dietterle et al.), pyrophosphate and iodine, see U.S. Pat. No. 5,948,235 (Aria et al.), diamino compounds, see U.S. Pat. No. 5,514,261 (Herklotz et al.), thiourea derivatives combined with alkanol amines, polyethylene imines, and alkoxylated aromatic alcohols, see U.S. Pat. No. 7,151,049 (Beica et al.), and thiourea combined with alkylsulfonic acid and a thio aromatic compound as a brightener, see U.S. Pat. No. 6,099,713 (Yananda et al.).
The current invention specifically addresses these tin-silver alloy electroplating problems by providing an electrolyte capable of high current density operation, at low silver concentrations, that operates near ambient room temperature. The invention also provides a low liquid agitation environment around the soluble anodes to further reduce the displacement of silver onto the anodes.